1. Field of the Invention
The present invention relates to a valve timing apparatus that is capable of controlling the timing of the opening and closing of the intake and exhaust valves in an engine, and more particularly, to a variable valve timing apparatus that is capable of adjusting the speeds upon upward and downward movements of upper and lower plungers through oil circulation to control the timing of the opening and closing of the intake and exhaust valves in an engine.
2. Background of the Related Art
Recently, the demand for eco-friendly, high efficiency engines has been increased, so that various improvements in the performance of the engines are provided, and further, optimal parts as well as control technologies capable of ensuring high efficiency in engines have been made and developed.
So as to improve the performance of engines, especially, intake and exhaust conditions should be optimized, and accordingly, various technologies have been proposed to optimize the timing of the opening and closing of intake and exhaust valves in the engine through the adjustment of the different timing of the valves caused by the operating speed of the engine.
Like this, the timing of the opening and closing of the intake and exhaust valves is varied in accordance with the operating speed and load of the engine, thereby optimizing the heat efficiency and output of the engine and improving the fuel efficiency. The above-mentioned technology is applied to a variable valve timing apparatus of an engine.
The variable valve timing apparatus generally advances or retards the timing of the opening of the valves through the variation of the phase of a cam. In the existing engines, the control of the timing of the opening and closing of the valves is conducted by using a combustion driving system through a cam profile having fixed mechanical valve timing.
In case of the conventional variable valve timing apparatus, however, it is impossible to vary the timing of the opening and closing of the valves during the operation of the engine unless the cam is changed, and further, a helical gear type, a torsional spline type, a vane type, or an electro-hydraulic type cam phase varying mechanism should be mounted on the front end portion of the camshaft, thereby making the configuration very complicated and also making the size of a cylinder head portion bulky.
In case of another conventional variable valve timing apparatus, a valve lift mechanism is changed in configuration to vary the opening keeping time of the valves or the timing of the opening and closing timing of the valves, thereby unfortunately making the configuration very complicated and also making the volume of a cylinder head portion bulky.